How to measure solar panel output

How to calculate solar panel output is a common question from homeowners. It makes sense, considering the impact solar panel production has on the overall system. Exactly how to calculate the power output of your solar panels involves a few variables and steps.

Some of the most important factors in figuring out your solar panel output include:

  • Your solar panels’ efficiency
  • Location (how much sunlight shines on your solar panels)
  • Which direction your solar panels are facing

Of course, there are other variables that can change the final output number as well, but the three above are the main contributors. We’ll go over each of these in this post, as well as give a formula for exactly how to calculate solar panel output.

How to Measure Solar Panel Output

What are Standard Test Conditions?

A good place to start is by understanding the parameters that earn a solar panel its wattage rating. How many watts your solar panel is able to produce might be anywhere between 250 watts and 370 watts.

Does this mean that your system will generate that exact amount all of the time? Not really. That’s where those variables come in. But a solar panel efficiency number is a gauge of how many watts your solar panel is capable of producing in ideal conditions.

These ideal conditions are simulated in a laboratory where solar panels are tested, known as Standard Test Conditions (STC). Standard Test Conditions for solar panel wattage would mean your solar panel is operating at 77 degrees Fahrenheit while there are 1000 watts of sunlight per square meter hitting the panel.

So, under these ideal conditions, a 250-watt solar panel will produce 250 watts of electricity. This standard is a good way to ensure that all solar panels meet specific criteria when they’re manufactured. As for how well they perform in the wild, let’s get into some of those variables.

Solar Panel Efficiency

What exactly is solar panel efficiency? While wattage can tell you what your solar panel is capable of producing under ideal conditions, efficiency tells you how much sunlight your solar panel is capable of converting into electricity you can use in your home.

For example, if your solar panel has an efficiency rating of 13 percent, that means that 13 percent of the sunlight striking your solar panel will be transformed into the power you need to toast bread or do a load of laundry.

Solar panel efficiency can be affected by a few variables itself, which can either subdue or boost it. Within the solar cells themselves, there can be variation in efficiency based on how reflective the cells are. Less reflective cells can collect more sunlight and use it rather than bouncing it back out to space.

The area around your rooftop solar panel system can also change your efficiency numbers. The most common environmental factors that can subdue efficiency are:

  • Shading from nearby trees or other buildings
  • Excessive cloud coverage
  • Excessive dirt, dust, and pollution
  • Thick layers of snow

There are some things to note about each of these. Shading is generally a fairly obvious efficiency blocker and should be avoided if at all possible. Trimming trees and positioning solar panels to avoid shading from other nearby structures will help.

Cloud coverage does not mean that absolutely no sunlight will make it to your solar panels, but the amount will obviously be reduced.

Dirt, dust, and pollution can degrade solar panel efficiency over time. Rainfall is a natural and easy way of cleaning them off. If you live in a particularly arid region where rainfall is scarce and dust is abundant, you can clean your solar panels yourself or hire someone to do it for you.

While it’s true that too much heavy snow can lower efficiency, some snow is actually a good thing because any dust, dirt, and pollution will cling to it and slide off the slick panels when the snow melts. Also, like most electronic equipment, solar panels operate well in cooler conditions.

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How location and direction affect solar panel output

Where you live and your solar panels

Where you live has a big impact on how much energy your solar panels are capable of producing. That’s why solar was first put to use in sunny places like the U.S. Southwest.

The more access to sun your solar panels have, the more they can produce. While some states in northern latitudes get less than 4 hours of sunlight per day on average, others can get more than 7.5 hours.

Obviously, the more sun, the better. But it’s still possible to enjoy the benefits of solar even in places up north.

The direction your solar panels are facing

This is another variable in solar output that is closely related to the general location of your home and also any potential shading factors.

The general idea here is fairly intuitive as well. If your solar panels are placed in a direction that receives the most sunlight, you give them the best chance to capitalize on that solar energy.

Whether the best direction for your solar panels is south or west depends on a few differences in both the physical space or your rooftop and the surrounding environment, as well as how your utility structures its electricity rates. Read more about the best position for your solar panels here.

How to Calculate Solar Panel Output

Here’s a simple formula for calculating your solar panel’s power output.

Solar panel watts x average hours of sunlight x 75% = daily watt-hours

As an example, let’s say you have 250-watt solar panels and live in a place where you get 5 hours of sunlight per day. What’s that 75 percent for? That’s to account for all those variables we've been going over.

250 watts x 5 hours x .75 = 937.5 daily watt hours

To translate this into the more familiar kilowatt hours you’re used to seeing on your electricity bill, simply divide by 1000.

937.5 / 1000 = 0.937

To round up and make it pretty, that’s 0.94 kilowatt-hours per solar panel.

Of course, you don’t have to do this math yourself. Vivint Solar experts will help you through these calculations to find the optimal solar panels for your home. We do this every day and we think it’s fun. Since we get a lot of questions about how to calculate solar panel output, we decided to share it with you.

If you have any more questions, feel free to reach out.

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